Passivity Based Robust Controller Design for 6-DOF Stewart Platform Manipulator Used in 4D Ultrasound Imaging

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Addis Ababa University


Stewart platform manipulators are six degrees of freedom robots having high structural rigidity, positioning accuracy, and are preferred over serial robots for applications such as robotic surgery, robotic ultrasound, and so on. However, the absence of robust controllers, their nonlinear dynamics and uncertainties due to model inaccuracies, parameter variations, and external disturbance, has limited the real application of the manipulators. Passivity based integral sliding mode control is one of the techniques available to design robust controllers and has been implemented for trajectory tracking of the Stewart platform. However, the robustness of passivity based integral sliding mode control has been achieved at the result of high-frequency switching of discontinuous control signal. The discontinuity in the feedback control is undesirable for practical applications because of chattering which causes the wear and tear of the mechanical actuators. In this thesis, the integral sliding mode control law is modified by replacing the discontinuous control part with super twisting control, and proportional derivative plus control is designed based on the passivation principle so that the closed-loop system becomes asymptotically stable. Particle swarm optimization is used to determine the optimal gain matrices values of passivity-based smooth integral sliding mode controller using an integral time absolute error as an optimization problem. The passivity-based smooth integral sliding mode controller has implemented using MATLAB/Simulink, and the result has been compared with a passivity-based integral sliding mode controller with different operating conditions. The simulation results show that for the case of a platform with load and disturbance together, the tracking error for six legs achieved by passivity based integral sliding mode controller and passivity-based smooth integral sliding mode controller was 0.1712, 0.0052, 0.1557, 0.0290, 0.0021, 0.0145 and 0.0005, 0.0018, 0.0023, 0.0017, 0.0017, 0.0006 meters respectively. The simulation results have shown that the designed passivity based smooth integral sliding mode controller performs better in chattering elimination and avoiding uncertainties due to load variation and variable disturbance.



Passivity based smooth integral sliding mode control, PD+ control, super twisting control, integral sliding mode control, particle swarm optimization algorithm, Stewart platform manipulator